severance pay mistake

Following a major merger, on Sept 2019, I was presented and I accepted in good faith with what I thought was a generous severance package stating what I will receive in terms of pay starting April 2020 after I complete for 6 months an office shutdown and transition of functions overseas successfully. I have been 21 years in the company. Comes April, the severance paychecks were of a much lower than expected amount. After contacting HR an internal review was made and it was deemed that a clerical error was made at the time of the offer and I was really only entitled to roughly 50% of what was presented on paper. I have worked relentlessly to complete my last 6 months task and have not sought employment opportunities I may have otherwise. Is the initial agreement binding ? do I have a case against my former employer ?